Resistance temperature detector working principle

The term RTD stands for Resistance Temperature Detector. This sensor is also known as Resistance thermometer. This sensor is used to measure the temperature. An RTD  is a temperature sensor that operates on the measurement principle that a material’s electrical resistance changes with temperature. The relationship between an RTD’s resistance and the surrounding temperature is highly predictable, allowing for accurate and consistent temperature measurement. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD’s resistance can be calculated, and the temperature can be determined. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD’s resistance can be calculated, and the temperature can be determined.

Resistance Temperature Detector working principle

RTDs work on a basic correlation between metals and temperature. As the temperature of a metal increases, the metal’s resistance to the flow of electricity increases. Similarly, as the temperature of the RTD resistance element increases, the electrical resistance, measured in ohms (Ω), increases. RTD elements are commonly specified according to their resistance in ohms at zero degrees Celsius . The most common RTD specification is 100 Ω, which means that at degree centigrade the RTD element should demonstrate 100 Ω of resistance. RTD resistance ratio The term “resistance ratio” describes the average slope of temperature vs. resistance as the RTD temperature changes from 0°C to +100°C. The expression for resistance ratio is: (R100-R0) / R0  Where: R100 = RTD Resistance at 100°C. R0 = RTD Resistance at 0°C. Resistance ratio is affected by the type and purity of the metal used to make the RTD. In general, RTDs that have a high R0 value combined with a high resistance ratio are easier to measure accurately, but other characteristics of the metal used in the resistance wire still affect the inherent accuracy of the RTD. Types Of RTD Platinum 100 ohm to 1000 ohm and Nickel 100 ohm to 1000 ohm Platinum is the most commonly used metal for RTD elements due to a few factors, including its
  • Chemical inertness,
  • Nearly linear temperature versus resistance relationship,
  • Temperature coefficient of resistance that is large enough to give readily measurable resistance changes with temperature and
  • Stability (in that its temperature resistance does not drastically change with time).
  Other metals that are less frequently used as the resistor elements in an RTD include nickel, copper and Balco. RTD elements are typically in one of three configurations:
  • Thin film RTDs are mass-produced and cost less than the other RTD types. They are smaller, and have a faster response time than the others, which is desirable in many applications. They are made by depositing a thin pathway of platinum on a ceramic base.
The manufacturer adjusts the resistance at 0°C by opening parallel shunts in the pathway with a laser beam. The more shunts are opened, the higher is the resistance at 0°C. Thin film RTDs are not as accurate as the other types because: The R0 resistance cannot be adjusted as precisely as in the other types.
  • The ceramic base and platinum coating have slightly different expansion rates. This creates a strain error at higher temperatures.
  • Because thin film RTDs are smaller, the RTD excitation current causes a slightly higher error due to RTD self-heating.
  • Wire wound RTD, a resistance wire is wound around a non-conducting core, which is usually made of ceramic. The sensor maker carefully trims the length of resistance wire to achieve the specified resistance at 0°C. This is called the “R0” resistance.
Next, lead wires are attached to the resistance wire, and then a glass or ceramic coating is applied over the wire for protection. As temperature increases, the length of resistance wire increases slightly. Care must be taken in the design to ensure that the resistance wire does not twist or otherwise deform as temperature increases. This is because mechanical strain causes a change in wire resistance. Laboratory-grade RTDs used by calibration and standards laboratories eliminate this source of error by loosely winding resistance wire around a non-conducting support structure. This type of RTD can be extremely accurate, but is fragile and not suited for most industrial applications.
  • In a coiled element RTD, the resistance wire is rolled into small coils, which loosely fit into a ceramic form that is then filled with non-conductive powder. The resistance wire is free to expand and contract as temperature changes, minimizing error caused by mechanical strain. The powder increases the rate of heat transfer into the coils, thereby improving the response time. Coiled element RTDs are usually protected by a metal sheath and are used in industrial applications.


Why are RTDs sometimes called 2, 3 or 4 wire RTD

 

A simple rule of thumb is that the more wires an RTD has the more accurate it is. The entire RTD assembly is not platinum. Among other issues, constructing an RTD in that manner would for most purposes be prohibitively expensive. As a result, only the small RTD element itself is made of platinum. As a practical matter the resistance value of the RTD element would be useless without a means to communicate that resistance to an instrument. Accordingly, insulated copper wires typically connect the RTD element to the measuring instrument.

Like platinum, copper has a resistance value. Resistance along the copper lead wires can impact the resistance measurement determined by the instrument connected to the RTD. Two wire RTDs do not have a practical means for accounting for the resistance associated with the copper lead wires which reduces the extent to which the resistance measured can be accurately correlated to the temperature of the RTD element. As a result, two wire RTDs are least commonly specified and are generally used where only an approximate value for temperature is needed.

Three wire RTDs are the most common specification for industrial applications. Three wire RTDs normally use a Wheatstone bridge measurement circuit to compensate for the lead wire resistance as shown below

Three Wire RTD

In a 3 wire RTD configuration, Wires “A” & “B” should be close to the same length. These lengths are significant because the intention of the Wheatstone bridge is to make the impedances of wires A and B, each acting as an opposite leg of the bridge, cancel the other out, leaving Wire “C” to act as a sense lead carrying a very small current.

4 Wire RTD

RTDs are even more accurate than their 3 wire RTD counterparts because they can completely compensate for the resistance of the wires without having to pay particular attention to the length of each of the wires. This can provide significantly increased accuracy at the relatively low cost of increased copper extension wire.

What are the common components of an RTD?
  1. RTD platinum resistance element: This is the actual temperature sensing portion of the RTD. Elements range in length from 1/8″ to 3″. There are many options. The standard temperature coefficient is an alpha of .00385 and the standard resistance is 100 Ω at 0 C.2. RTD Outside diameter: The most common outside diameter in the US or 6mm (.236″) for non-US applications. However, outside diameters range from 0.063″ to 0.500″RTD Tubing Material: 316 Stainless steel is commonly used for assemblies up to 500 Degree F. Above 500 degree F it is advisable to use Inconel 600.3. RTD Process Connection: Process connection fittings include all standard fittings used with thermocouples (i.e. compression, welded, spring-loaded, etc.). 4. RTD Wire Configuration: RTDs are available in 2, 3 and 4 wire configurations. 3 wire configurations are the most common for industrial applications. Teflon and fiberglass are the standard wire insulation materials. Teflon is moisture resistant and can be used up to 400-degree F. Fiberglass can be used up to 1000-degree F. 5. RTD cold end termination: RTDs can terminate on the cold end with plugs, bare wires, terminal heads and any of the reference junctions common to thermocouples.
 





Applications :

  • RTD sensor is used in automotive to measure the engine temperature, an oil level sensor, intake air temperature sensors. In communication and instrumentation for sensing the over the temperature of amplifiers, transistor gain stabilizers, etc.
  • RTD is used in power electronics, computer, consumer electronics, food handling and processing, industrial electronics, medical electronics, military, and aerospace.

Range :-  − 328 to 1532 ° F ( − 200 to 850 ° C) is IEC standard for platinum RTDs and  − 330 to600 ° F ( − 200 to 320 ° C) for nickel RTDs, Practical applications are usually limited to − 328 to 1000 ° F ( − 200 to 537 ° C)

Linearity: Platinum and copper are more linear; nickel and nickel/iron (Balco TM ) are less so.The use of gold and silver RTDs is limited to cryogenic temperatures.

Stability: Zero and span drift is usually within 0.1% of span for a 6-month period (and frequently longer).

Price :- Elements alone range from about $35 to $80; RTD assemblies, including thermowells,are from $100 to $250. 

Advantage :-

  • They are most stable.
  • They are most accurate.
  • Linearity is higher compare to thermocouples.

Limitations :- 

  • They are passive i.e. current source is needed.
  • They are very expensive.
  • Self heating.
  • Less rugged compare to thermocouples.
  • Small change in resistance with temperature.

Read Next :- 

Thermocouple Working Principle

K type thermocouple

Difference Between RTD and Thermocouple and Thermistor

 

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